Weird Central Heating Problem

[PhilWis]

Can anyone help with this because my gas fitter can't get to the bottom of it.

Have an indirect system and have recently had old floor standing boiler (must have been about 20 years old) with a wall mounted Glow Worm. Also had new pump, valves, timer and some pipe work as the boiler was moved from an understairs cupboard to an outer wall in the utility room.

Two major problems.

1. When using the HW programme only, the pump runs pretty continuously throughout the day even when the HW has reached its temperature as set by the cylinder thermostat. We thought the thermostat might be faulty but replacing this has not made any difference.

2. When the CH is swithed on, the downstairs radiators take an absolute age to warm up. However, if the HW progamme is turned off then they heat normally and quickly.

Feeling is the two are related and that downstairs radiators are not working properly because the pump is still using 50% of its capacity to heat the water when it isn't necessary.

Can't seem to get to the bottom of the problem. Anyone have any bright ideas please?

Thanks

 

[chrishutt]

I think your analysis is along the right lines. Has this problem dated from the alterations to the system, or is it more recent? Do you have 1 or 2 motorised valves?

I suspect a wiring fault. You can check how the wiring should be by looking at the wiring diagrams on the Honeywell site. S plan has 2 motorised valves, Y plan has 1.

If the work was recent, you ought to insist that the installers sort it out, although if they're incompetent that might not be so wise.

 

[PhilWis]

The problem has only arisen since the alterations.

There is only one motorised valve.

The wiring has been checked and is apparently ok.

The installer is completely friendly (obviously CORGI) and willing to sort the problem but just can't understand what it is.

Thanks

 

[slippyr4]

you've got one valve, so it's probably a mid position valve. There is considerably less flow resistance on the HW circuit than the CH. Hence when you are wanting both hot water and CH, you're not getting a lot of CH cos it's just baking the HW all the time. This, in itself, isn't a huge problem, if your cylinder is a high recovery one and can reheat quickly.

Your cylinder stat should be turning off the HW when it's up to temperature. You say you've replaced it, but have you checked that it is working? Somewhere the wiring is screwed between the cylinder stat and the mid position valve, and the result is that the HW satisfied signal is not getting through.

Check, and double check, the wiring diagrams. use a multimeter to verify that the wiring matches the diagram and there are no mistakes.

The wiring has been checked and is apparently ok.

It's not ok.

 

[PhilWis]

Thanks - will get it checked again.

 

[chrishutt]

Just to add to what slippyr4 says above, which I agree with completely, the wiring for a mid position valve is relatively complicated and a little counter-intuitive, so there's always a good chance of someone getting it wrong.

Did you find the Y plan diagram from Honeywell? Well worth studying if you have a basic understanding of electrics. Note in particular that the cylinder stat must have all 3 connections (including the "satisfied" terminal) and that a connection is taken from the HW off terminal of the programmer which links to the grey wire of the motorised valve, along with the cyl. stat satisfied connection. I bet this is where the fault is!

One other thing to be aware of is the need for the boiler flow temp. to be well above the thermostat setting on the cylinder, otherwise the cylinder stat will never be satisfied. As I understand it this isn't your problem since you seem to be saying that the cylinder stat switches off but flow continues to the cylinder.

 

[PhilWis]

Thanks Chrishutt.

The installer has used a Danfoss Wiring Centre to connect everything up. I've taken the cover off and had a look. It seems to accord with the wiring diagram required for a Fully Pumped System 3 Port Mid Position Valve and he's left his wiring notes aswell.

Re the boiler temp flow, the installer was wondering if this was the problem (having checked the wiring as he said he had). The cylinder and stat are in an upstairs airing cupboard whereas the boiler is in a downstairs utility room some way away. I suppose they must be 4-5 metres distance apart so I suppose the flow and return total distance would be about double this or am I misunderstanding?

The cylinder thermostat is set to 55C. What should I set the boiler thermostat to to avoid any problems? You can't actually set it to an actual temp although there's an LCD which reads off what it achieves when alight. If this is set to say 65C then should this be sufficient to satisfy the cylinder?

 

[chrishutt]

Height of cylinder above boiler doesn't matter. Temperature settings that you mention should be OK, but I would set boiler thermostat a bit higher to speed up heat transfer. Also make sure the cylinder stat isn't attached too low to the cylinder - should be between 1/4 and 1/3 up from bottom of cyl. - about mid way between flow and return connections to coil.

 

[wilhelm]

w plan?

 

[PhilWis]

can I post one?

 

[wilhelm]

post what ya want.

 

[chrishutt]

Fully Pumped System 3 Port Mid Position Valve

So must be Y plan?

 

[wilhelm]

never assume anything??

 

[chrishutt]

Yeh, well.... you have to assume some things, or we'd never be able to suggest anything.

 

[PhilWis]

sorry you've both lost me!